Ceiling luminaries are widely used in commercial installations to provide both overall lighting and concentrated area lighting to the rooms in which they are placed. For many years, little effort was made to reduce high angle glare from such fixtures or to spread the light or shape the light pattern emitted from the lamps in such fixtures. In addition, the lamps themselves frequently were directly visible.
In recent years, however, it has become increasingly important to engineer such ceiling light fixtures in a manner to more evenly distribute the light emanating from the fixtures and to eliminate, as much as possible, the high angle light which causes uncomfortable glare to persons located within the room in which such fixtures are located. Attempts also have been made to develop lenses which hide the lamp, that is which spread the light emanating from the luminaire in a manner as to obscure the shape of the lamp located within the luminaire when the luminaire is viewed from below.
Simple diffusers are widely used to hide the lamps, and diffusers generally perform this function satisfactorily. Such diffusers, however, tend to be low in efficiency and accomplish little or nothing in glare control (that is elimination of high angle light rays). Prismatic lenses of a variety of different designs have been developed to provide reasonably good lamp hiding power with high efficiency, but such prismatic lenses produce glare due to undesirably high brightness close to the horizontal.
Attempts have been made at designing luminaires which employ refractive lenses designed to eliminate glare (high angle light). Such lenses are disclosed in the U.S. Pat. Nos. 2,434,049 and 3,763,369 to Nordquist and Lewin. The lens of the Nordquist patent has a number of interconnected elongated lens sections extending transversely across the width of a fluorescent light fixture. Each of the lens sections is formed with a convex light emergent portion, and the lens sections are provided with upwardly tapering ribs of opaque or reflective material designed to block light above certain angles longitudinally of the fixtures. The result of the Nordquist lens is to minimize endwise glare from the fixture. Transverse glare, however, is relatively unaffected.
A refractive grid designed to minimize glare in all directions is disclosed in the Lewin Pat. No. 3,763,369. The glare reduction capabilities of the lens of this patent and the manner of measuring glare and luminaire efficiency are discussed in detail in an article appearing in the Journal of IES, April 1973, on Pages 209 through 214 entitled "NEW CONCEPTS IN DIRECT GLARE CONTROL", by Ian Lewin. The refractive grid of Lewin's U.S. Pat. No, 3,763,369 provides fairly good glare control although some high angle light rays are emitted from the refractive grid of the U.S. Pat. No. 3,763,369.
The grid or lens of Lewin's U.S. Pat. No. 3,763,369 consists of a large number of concavo-convex circular lenses interconnected in a honeycomb-like pattern to form the refractive grid. The top surface of each of these interconnected lenses is a concave light incident surface, and the convex side of each of the lenses comprises the light emergent surface. The radii of the two surfaces are slightly different, with a slightly smaller radius being employed for the convex light emergent surface than for the concave light incident surface. The individual lenses are joined together with a relatively high angle screening surface to block high angle light from passing through the lens grid.
While the lens of U.S. Pat. No. 3,763,369 substantially reduces glare, it does not provide very good lamp hiding power. Light rays passing through the concave light incident surface on the top of the lens and emerging from the convex bottom surface of the lens undergo nearly self-cancelling refraction at the two surfaces, so that a viewer sees the relatively undisturbed rays just as if he were looking through a window. Consequently, the lamp is clearly defined and visible to a viewer standing below the lens and looking up into it. To improve the light hiding capabilities of luminaries using a lens of the type disclosed in the U.S. Pat. No. 3,763,369 a diffuser is placed over the lens or an auxiliary refractor sometimes is placed between the lamp and the lens to achieve a more even brightness. This is done, however, at substantial cost and with reduced efficiency from the luminaire.
It is desirable to provide a lens for a luminaire which significantly reduces undesirable glare, provides high efficiency and obscures (hides) the lamp contained in the luminaire to provide an appearance of even brightness.